[DuncanWaring]

... nitrogen would render inmates unconscious within ten seconds...

I doubt that.

If it were true, nobody would be able to hold their breath for more than ten seconds.

[thackney]

Holding your breath retains the oxygen already present in your lungs. Breathing in pure nitrogen replaces that oxygen.

That said, it still sounds too quick...

[taxcontrol]

Nitrogen Asphyxiation is FAST. So fast that most people who are exposed are unable to help themselves or even leave the environment. 10 seconds maybe, 30 seconds more likely.

[thackney]

There is still significant oxygen in the air you would normally exhale. This method would take that quickly to zero percent O2.

For comparison:
http://www.normalbreathing.com/patterns-oxygen-extraction.php

Composition of outer or inhaled air:
O2 - Oxygen 158 mm Hg (20.9%)
CO2 - Carbon dioxide 0.3 mm Hg (0.04%)
H2O - Water 5.7 mm Hg (0.75%)
N2 - Nitrogen 596 mm Hg (78.4%)

Composition of the expired air:
O2 - Oxygen 116 mm Hg (15.3%)
CO2 - Carbon dioxide 32 mm Hg (4.2%)
H2O - Water 47 mm Hg (6.2%)
N2 - Nitrogen 565 mm Hg (74.3%)

[spokeshave]

Just add a little Nitrous Oxide gas to the mix....and they die laughing.